This invention is related to the treatment of a liquid drilling fluid or drilling mud used in the drilling of a borehole into the earth.
In the drilling of a borehole by rotary drilling techniques a drill bit is attached to the lower end of a drill string and the drill string is rotated and lowered to form a borehole in the earth. A drilling fluid is circulated through the borehole normally down the drill string to the bottom of the borehole and thence upward through the annulus to the surface of the earth. The drilling fluid may be either liquid or gaseous but usually is liquid and is commonly referred to as a drilling mud. The drilling mud may be either water base, oil base, or an emulsion.
The circulating drilling mud cools and lubricates the drilling bit and drill string, removes earth cuttings, referred to as "drilled solids", from the borehole, forms a filter cake on the borehole wall, and controls formation pressure. In order to best perform these functions, additives are included in the drilling mud to obtain desired rheological properties. During the progress of drilling it is necessary to monitor and treat the drilling mud to maintain these desired rheological properties.
Drilled solids tend to accumulate in the drilling mud and if not removed will seriously deteriorate the rheological properties thereof. For example, as pointed out in U.S. Pat. No. 3,766,997, to Joe K. Heilhecker et al., drilled solids increase the viscosity and density of the drilling fluid, reduce the carrying capacity of the fluid, promote poor filter cake qualities, and damage drilling equipment.
Hydrocyclones are used to treat drilling muds, particularly unweighted drilling muds, to remove drilled solids therefrom. A hydrocyclone is a separator having a generally conical separation chamber with an inlet disposed generally tangentially to the side of and adjacent the base of the cone of the chamber, with an axial underflow outlet located adjacent the apex of the cone, and with an axial overflow outlet located adjacent the base of the cone. The drilling mud is fed under pressure into the inlet and the pressure energy is converted into centrifugal force. The developed centrifugal forces multiply the settling velocities of the suspended solids, driving the larger and heavier particles outwardly toward the conical wall and downwardly into a centrifugally accelerating spiral along the wall to the underflow outlet, the solids discharge point at the apex of the cone. The liquid phase of the drilling mud, carrying the smaller and lighter drilled solids, moves inwardly and upwardly as a spiraling vortex to the axial overflow outlet adjacent the base of the cone.
In U.S. Pat. No. 3,025,965, to William E. Bergman et al., there is disclosed a hydraulic cyclone separation system for separating a portion of the larger and heavier solids which are temporarily suspended in a liquid rotary well drilling mud from the remainder of the mud. It was there noted as follows: In the prior art of hydraulic cyclone separation of solids from well drilling muds considerable difficulty has been experienced. The mud is too concentrated, and we have found it needs dilution with water. Upon shutdown of the system between periods of use, the mud settles into a solid cake on the walls of the mud pump and mud lines, making it impossible to start up the unit until sufficient parts are disassembled and cleaned out to permit the mud pump to operate again. The walls of the hydraulic cyclone chamber are rapidly worn away by abrasion of the heavy undiluted mud containing abrasive solids and clays without dilution water. The invention of Bergman et al. is directed to solving these problems by providing a plurality of pumps of which at least one pumps mud and at least another pumps only water. Dilution water is run into the mud going to the hydrocyclone to reduce the abrasion thereof and the hydrocyclone cone is made out of Tungsten carbide to make it resistant to abrasion. Water is pumped from the water pump through the feed pump and lines before a shutdown to eliminate the deposit of solid mud therein.
In the before-mentioned Heilhecker et al. patent there is disclosed a system for treating a drilling fluid being circulated in a well and containing a fine-sized particulate weighting material and drilled solids wherein the drilling fluid is passed through a first vibrating screen which removes a portion of the drilled solids and then through centrifugal separating means to separate the drilling fluid into a low density effluent and into a high density underflow slurry. The effluent is returned to the drilling fluid system and the underflow slurry is further processed through a second vibrating screen. The second vibrating screen is substantially finer than the first vibrating screen and functions to remove additional drilled solids. Material passing through the second vibrating screen which includes most of the weighting material and the underflow slurry is returned to the drilling fluid system.